The gradual establishment of cell commitments during the early stages of chick development

The early development of the chick (Stages I-XIII E.G&K) can be regarded as an ideal model for epigenetic development and for the study of the forces and factors involved in the establishment of cellular heterogeneity and the imprinting of polarity. It seems that a physical vectorial force is utilized to imprint upon the cleaving radial-symmetric multilayered blastodisc, a postero-anterior metabolic gradient, which is translated into the first morphogenetic phenomenon - a polarized cell shedding, causing the formation of the area pellucida. In a freshly laid egg, the Stage X blastoderm despite its radial appearance, has a concealed bilateral symmetry which is not susceptible any longer to spatial changes, but nevertheless is still labile and can be changed by several other drastic experimental procedures. This means that the individual cells are still pluripotential and not yet specifically committed. During the first 10 h of incubation a second polar morphogenetic process, the formation of Hyp, occurs following the same orientation of the previous cell shedding and thus stressing and enforcing the postero-anterior axis of bilateral symmetry. In the resulting double layered blastula, each layer separately expresses its own polar characteristics along the mutual axis of symmetry. Even at Stage XIII the Ep still remains a totipotential system which can regenerate a normal Hyp, and its polarity is probably in the form of a labile gradient field of competence for PS formation. The Hyp, on the other hand, has sorted out from the single layered Stage X blastoderm and is more specialized, as demonstrated by several developmental, metabolic, and immunologic criteria. Its only developmental potential is the polar ability to induce a PS in a competent Ep. The Hyp was found to contain two different cell populations, of which only one, of marginal zone origin, has the PS-inducing capacity. Even after the PS has started to form, the induction is still uncompleted and has to go on until the PS is more than half its full length. In order to render possible normal PS formation, at least one of the two layers has to retain its polarity. In this respect the polarity of the competent Ep seems to be more crucial than that of the inductive Hyp. Polarity and inductivity of the Hyp can be dissociated from each other, and a disorganized Hyp can still induce a normal PS as long as the Ep retains its polarity.